An application entitled, xe2x80x9cAPPARATUS AND METHOD FOR OPTICAL PATTERN DETECTIONxe2x80x9d filed on the same date as this application having the same inventor and assigned to the same assignee as this application is hereby incorporated by reference.
The invention relates to diffraction gratings and, in particular, to the temperature-compensation of diffraction gratings.
Diffraction gratings are passive optical devices that diffract a collimated light beam in specific directions according to various parameters, including the angle of incidence of the beam on the grating, the optical wavelength of the beam, the spacing of the lines that form the grating, and the blaze angle or Bragg angle of the grating. Although relatively simple devices, diffraction gratings find widespread and important application in various technologies. For example, in the communications industry, gratings are employed to divert optical signals in optical add/drop/multiplexers (OADMs) in optical crossconnects (OXCs), and other telecommunications components. Gratings redirect light within optical spectrum analyzers, in signal measurement and processing equipment, and in other signal measurement and processing equipment employed within the medical and biomedical fields.
The geometry of a diffraction grating is a function of its temperature, with the grating expanding and contracting with temperature variations. Additionally, a diffraction grating""s thermoelectric properties change with temperature. Because these grating characteristics are critical to a grating""s proper operation, temperature effects must be compensated for in order to insure the proper operation of the grating and the equipment which rely upon it.
One approach to compensating for the effects of the varying temperature of a grating is to employ a thermocouple in a closed loop control system. In such as system the thermocouple, attached to the grating""s housing, senses temperature variations and provides feedback to a controller which operates a thermoelectric cooler or fan in response to the signal from the thermocouple. This approach is somewhat indirect, with no sensing of an actual temperature-dependent wavelength shift, which introduces the potential for misdirected or inadequate compensation. Additionally, there is an inherent lag built in to the system, since the thermocouple, attached to the grating housing, responds only after the housing reaches the temperature of the grating inside and the electronic control system requires additional time to respond to changes in the thermocouple. Such a system is also relatively complex, with the cost and reliability issues attendant to complexity.
Another approach to compensating for the deleterious effects of temperature variation on diffraction gratings involves attaching the gratings to substrates which have substantially equal and opposite thermal coefficients of expansion. See U.S. Pat. No. 5,694,503 to Fleming et. al., issued Dec. 2, 1997, which is hereby incorporated by reference. One of the disadvantages to this approach is that, again, it is somewhat indirect, with no sensing of an actual temperature-dependent wavelength shift. Additionally, this approach may require the use of somewhat exotic and expensive materials, and it is difficult to match materials with equal and opposite coefficients of thermal expansion, particularly since those coefficients may vary from lot to lot of the material.
An apparatus and method for diffraction grating temperature-compensation that employs a direct indication of a temperature-dependent wavelength shift would therefore be highly desirable.
In accordance with the principles of the present invention a temperature-compensated optical grating is fed an optical probe signal of a predetermined wavelength. An optical detector is positioned substantially in the region to which the probe signal would be deflected at a calibrated or xe2x80x9czeroxe2x80x9d position. The optical detector is also positioned to determine the deflection of the probe signal from the zero position and to pass the deflection information along to a controller. The controller is configured to adjust a temperature adjustment device, such as a thermo-electric-cooler, to return the temperature of the grating to a preferred value associated with the calibrated, or zero, deflection value of the probe beam. The preferred temperature of the grating, and corresponding zero deflection value of the probe beam, may be, for example, at xe2x80x9croom temperaturexe2x80x9d.
Various optical components may advantageously employ a temperature-compensated diffraction grating in accordance with the principles of the present invention. For example, an optical crossconnect may employ such a diffraction grating to insure reliable operation in spite of temperature drifts. Similarly, an optical add/drop multiplexer may employ a temperature-compensated diffraction grating in accordance with the principles of the present invention in order to compensate for detected differences in deflection angle shifts due to temperature variations.
An array of photodetectors may be employed to provide a measure of a probe signal""s temperature dependent deflection, or, alternatively, an integrated optical detector which requires as few as one photodetectors may be employed to provide feedback to the controller.
Although there a variety of diffraction gratings, for the clarity and convenience of description the present invention will be discussed generally in terms of a reflection grating. As will be recognized by those skilled in the art, the principles discussed herein may be applied to gratings other than reflection gratings.